Many conventional computer systems include interfaces to multiple peripheral devices. Some peripherals, such as a keyboard, a mouse and a monitor are generally necessary. Other peripherals are optional. The optional peripherals in any particular system depend on the needs of the particular user. Optional peripherals may include speakers, digitizing pads, printers, scanners, modems, external hard drives, memory cards, camera interfaces, and the like. Many different types of interfaces (e.g. RS-232 serial port, parallel printer port, game port, etc.) have been developed in order to connect peripheral devices to computer systems. In many systems each peripheral device requires its own dedicated interface. Such a technique for connecting peripherals is adequate when the number of peripheral devices does not exceed the number of interfaces available; however, once all the interfaces in such a system are in use it is not possible to add any more peripherals to the system. This limitation, among other, led to the development of the Universal Serial Bus (USB) interface.
USB is a serial digital interface that can provide up to 127 cascading interface ports controllable through a single USB interface on a computer. The USB interface eliminates the need for a new interface each time a peripheral is added to a system. However, the USB interface still generally requires a cable to connect a peripheral to a computer. While a USB cable is relatively small, the connection of multiple peripherals to a computer can quickly create a cabling mess.
One method of eliminating cables and going wireless uses a narrowband wireless transceiver connected to a computer. The peripheral then communicates with the wireless transceiver using a dedicated radio frequency (RF) channel to transmit information, usually at 49 MHz. However, a narrowband wireless system is prone to interference from other wireless devices and it is easy for an unauthorized user to tap into such a connection. Since narrowband transmission is less than ideal for adding wireless peripherals to a computer, other RF techniques have been developed. One of these other techniques is called direct sequence spread spectrum (DSSS).
The advantage of using a DSSS connection is that DSSS uses a more robust signal that is less susceptible to interference or eavesdropping versus a narrowband system. DSSS works by first encoding a data stream to be transmitted by using a multi-bit (referred to as multi-chip) pseudo-noise code (PN-Code) to replace each logical 1 and 0 of the data stream with either the PN-Code itself or the logical inverse of the PN-Code. The encoded data stream is then modulated onto an RF carrier and broadcast to the DSSS receiver.
DSSS systems can use a fixed-length PN-Code or they can use a varying-length PN-Code. However, DSSS systems are usually designed to use a fixed-length PN-Code in order to simplify the hardware needed to transmit and receive the data. For example, a well-known standard designated as IEEE 802.11 wireless LAN (WLAN) only uses a fixed-length 11-chip PN-Code called a Barker code. DSSS systems that are designed with varying-length PN-Codes are generally more costly to build and as such they are often not suitable for cost-sensitive applications.
Even though a fixed-length PN-Code keeps the complexity of a DSSS system to a minimum it does come at the cost of system flexibility. This is because the length of the PN-Code directly affects the transmission range and the data rate of the DSSS system—with longer PN-Codes allowing for greater transmission range and shorter PN-Codes allowing for greater data throughput. Therefore a fixed length PN-Code DSSS system is generally unable to adaptively alter the transmission range or the data throughput of the system.